This invention relates to thin-walled bearings, in particular but not essentially for automotive use.
Conventionally such a thin-walled bearing comprises a layer of bearing material bonded to the inner surface of a curved steel shell. The present invention further relates to a thin-walled bearing in which the bearing material is either aluminium based or copper based, and may be cast, sintered or roll bonded onto the steel shell. A typical thickness for the bearing layer is 250 micrometers.
It is also conventional that for a copper based bearing material, and sometimes for an aluminium based bearing material, an overlay, say of a thickness of 20 micrometers, is applied to the bearing layer, say by being electro-deposited on the bearing layer. The purpose of the overlay is to prevent the bearing material, especially when it includes copper, from seizing with a steel shaft journalled in the bearing; and/or to conform to the eccentricities of the shaft. Usually, in the operation of the bearing, the bearing layer becomes partially exposed through the overlay because of wear of the overlay, but the presence of even a partial overlay imparts the required properties to the bearing. Examples of overlay material which are employed are lead-10% tin, lead-7% indium and lead-tin-2-4% copper.
Properties required for both the bearing material, and the overlay material, are resistance to corrosion, and a high fatigue strength.
It is also known to electro-deposit a intermediate layer of nickel, upto two micrometers thick, on a cast layer of bearing material. Such an intermediate layer is provided to prevent bond failure between the bearing material and a subsequently electro-deposited overlay. A disadvantage of such bearings is that if a steel shaft wears away the overlay, so that the intermediate nickel layer is at least partially exposed, then seizure with the steel shaft can occur.
It is to thin-walled bearings having an overlay, and possibly also an intermediate nickel layer, as referred to above, that, in particular, the present invention relates.
It is an object of the present invention to provide a thin-walled bearing, for example for automotive use, which has a desirably long operating life; and which has a desirably close fit with a co-operating shaft for optimum performance.
In accordance with the present invention a thin-walled bearing including a layer of an aluminium based, or copper based, bearing material bonded to the inner surface of a curved steel shell, has an overlay, the overlay is of at least one of the elements lead, tin, indium, copper, nickel, zinc and antimony, and from a reaction of a flash less than two micrometers thick and of at least one of the elements iron, chromium, nickel, cobalt, gold, silver and copper, with the, or at least one constituent, element of the overlay, there is formed a modified, hard, wear-resistant, exposed surface portion of the overlay.
The flash is reacted, or combined, with the, or at least one constituent, element of the overlay so as to form at least one intermetallic compound; and/or to form at least one solid solution corresponding to, and possibly softer than, the intermetallic compound or compounds.
The modified, hard, wear-resistant, exposed surface portion of the overlay is also required to have seizure resistance, and fatigue strength not significantly lower than the unmodified major part of the overlay. The hardness of the surface portion is not conducive to it conforming to the eccentricities of a cooperating shaft, but because this portion is so thin its presence is not significantly detrimental to the effective conformability of the whole of the overlay with the modified, hard surface portion.
The flash may have combined, or reacted, in the required manner with the, or at least one constituent, element of the overlay by having been heated to a temperature which permits diffusion and alloying between the overlay and the flash at a desired rate.
It is unimportant if such good seizure resistant, and hard, wear-resistant, parts of the exposed surface portion of the overlay are discontinuous, possibly as a result of the portion being partially worn away during the operation of the bearing.
According to another aspect the present invention comprises a method of manufacturing a thin-walled bearing, which bearing includes a layer of an aluminium based, or copper based, bearing material bonded to the inner surface of a curved steel shell, and on the bearing layer is bonded an overlay of at least one of the elements lead, tin, indium, copper, nickel, zinc and antimony, and the method includes providing, in any convenient way, on the initially unmodified, exposed, surface portion of the overlay a flash less than two micrometers thick and of at least one of the elements iron, chromium, nickel, cobalt, gold, silver and copper, and reacting the material of the flash with the, or at least one constituent, element of the overlay, to form a modified hard, wear-resistant exposed surface portion of the overlay.